The reduction ratio of a rolled material can be computed from the speed of movement of the material before and after the rolling. As a means for measuring the speeds of movement of the material before and after the rolling, a contact type apparatus was often adopted in which measuring rolls or other measuring devices were brought in contact with strip coils on the inlet and outlet sides. However, recently, since the rolling speed has greatly increased and the necessity for a wet rolling using water or oil because of high-pressure operation has increased, the problem of a reduction of the measurement precision due to slip page has become serious, and therefore, a non-contact type apparatus in which magnetic marks are formed on the inlet and outlet sides and these marks are inspected in a non-contact manner has been developed. Apparatuses of this type are disclosed in Japanese Examined Utility Model No. 43-29667 and Japanese Unexamined Patent Publication No. 55-94711.
Japanese Examined Utility Model Publication No. 43-29667 discloses a basic apparatus in which the speed of movement of a magnetic mark on a material is measured before or after rolling. However, this apparatus is defective in that, when the material is moved at a high speed, a delay in operation occurs in either a magnetizer or a detector and an offset error inherent to the rolling operation cannot be avoided. As a means for overcoming this defect, the applicant of the instant application developed and proposed the measurement apparatus disclosed in Japanese Unexamined Patent Publication No. 55-94711. However, when this apparatus was put to practical use, it was found that this apparatus still leaves problems to be solved. Namely, in the actual rolling operation, since the material has a property such that the hardness on the inlet side is lower than the hardness on the outlet side, the intensity of the magnetic mark is proportionally reduced, and an erroneous operation is readily caused by noise, especially in the case of a soft material such as low-carbon steel for use in deep drawing, for which the demand has recently increased. Moreover, since the material on the inlet side is softer than on the outlet side and local deformation is readily caused by the winding motion of the roll through its travel, the gap, between the material and the detector is changed, which generates noise and worsens the S/N ratio, and in this case, of the comparison level of the detector is fixed, the error in the detection is enlarged.
It is an object of the present invention to provide a measurement apparatus in which the problems of the apparatus disclosed in Japanese Unexamined Patent Publication No. 55-94711 are solved and the reduction ratio can be precisely measured irrespective of the kind of material to be rolled.